The invention relates to a method for reducing nitrogen dioxide emissions from a motor vehicle having a lean burning internal combustion engine with a connected exhaust gas purification system comprising an SCR catalytic converter.
German patent document DE 10 2005 049 655 A1 discloses a known method for reducing the emission of nitrogen dioxide (NO2) in a lean burning internal combustion engine with an exhaust gas purification system having a catalytic converter with nitrogen oxide oxidation activity. In this method, the portion of materials is varied during the operation of the internal combustion engine, which compete with the oxidation of nitrogen monoxide (NO). As a result, the NO2 content in the exhaust gas downstream of the catalytic converter with NO oxidation activity takes on an amount preferred with the view to a following exhaust gas aftertreatment. The main focus is thus on a selective reduction of nitrogen oxides (NOx) carried out at an SCR catalytic converter or on an oxidation of soot particles deposited at a particle filter.
It is indeed avoided by the method that an unnecessary or disadvantageous excess of NO2 is formed with regard to the mentioned following exhaust gas aftertreatment processes, but a release of NOx in general and NO2 especially to the environment does not reduce reliably to a desired amount. An undesired emission of NOx and especially of NO2 at lower exhaust gas temperatures is especially not avoided in a reliable manner.
One object of the present invention to provide a method which achieves a reduction of the nitrogen dioxide emission of an internal combustion engine over a wide range of operations and especially with critical low exhaust gas temperatures in this regard.
This and other objects and advantages are achieved by the method according to the invention in which, above an operating temperature at which a noticeable reduction of nitrogen oxides with ammonia can take place in an SCR catalytic converter, an exhaust gas enriched with ammonia is supplied to an SCR catalytic converter, that has adsorption centers for nitrogen oxides.
The SCR catalytic converter, which is enabled for the catalyzation of a selective NOx reduction under oxidizing conditions by means of NH3, has adsorption centers where nitrogen oxides (NOx) can adsorb, so that they are withdrawn from the exhaust gas. This ability to absorb NOx affects the nitrogen oxides NO and/or NO2, while adsorption of other nitrogen oxides such as N2O, N2O3, N2O4 and other, especially polar materials, remains possible. The following text refers to “NOx”, if nitrogen monoxide (NO) and also one of the other mentioned nitrogen oxides can be affected. However, with regard to nitrogen dioxide (NO2), only this nitrogen oxide is affected.
Below the operating temperature referred to above, the exhaust gas supplied to the SCR catalytic converter is enriched with a material according to the invention, which adsorbs at least partially on the adsorption centers of the SCR catalytic converter, in such a manner that an adsorption of NOx is inhibited at these adsorption centers.
The device for enriching the exhaust gas with ammonia and/or a reagent enabled for splitting off ammonia is preferably an addition device, which can supply an externally provided reduction agent, such as ammonia (NH3), urea, ammonium carbamate or ammonium formiate, to the exhaust gas from the exterior. An addition device for aqueous urea solution is preferred, which can be injected into the exhaust gas. Due to a thermolysis and/or hydrolysis, NH3 is released from the urea in the exhaust gas at increased temperatures, with which a selective NOx reduction takes place at the SCR catalytic converter under oxidizing conditions. The mentioned device can however also be a reactive unit integrated in the exhaust gas purification system and flown through by the exhaust gas, which can generate ammonia from exhaust gas components contained per se in the exhaust gas or can generate exhaust gas components subsequently guided to the exhaust gas in a complete or partial manner. For this, a catalytic unit as for example a three-way or nitrogen storage catalytic converter is considered primarily, which can reduce supplied or stored nitrogen oxides to ammonia by means of exhaust gas components acting in a reducing manner.
In this connection, the operating temperature of the SCR catalytic converter referred to above, is meant to be a temperature above which a noticeable NOx reduction is enabled at the SCR catalytic converter with supplied NH3. The operating temperature can thereby be defined by a start-up temperature of the SCR catalytic converter, above which the SCR catalytic converter reaches a noticeable activity for the selective NOx reduction with NH3. The operating temperature can however also be characterized by a decomposition temperature, above which NH3 is released in a noticeable manner from the added reagent. In both cases, the conditions for a selective NOx reduction with NH3 in the SCR catalytic converter are not present or only to a small extent below the operating temperature. With the method according to the invention, a reduction especially of NO2 is enabled below and above the operating temperature.
It was discovered by the inventors that, at least with certain types of SCR catalytic converter, especially carbon monoxide (CO), carbon dioxide (CO2), hydrocarbons (HC), or water (H2O) and/or other materials can adsorb at the adsorption centers for NOx in such a manner that an NOx adsorption in inhibited at these adsorption centers. The SCR catalytic converter is typically formed in such a manner that at least with comparatively low temperatures, the relevant adsorption centers can bind one or several of the mentioned or further materials in an adsorptive manner with a comparable or even stronger binding strength than NOx. A prevention or at least a strong reduction of the NOx adsorption can however also be achieved alone in that the corresponding material is offered in a distinct excess compared to NOx, and thus an occupancy of the adsorption centers with this material takes place in a preferred manner.
It was discovered by the inventors that, with a heating of the SCR catalytic converter, previously adsorbed and thus collected NOx is released especially in the form of NO2 when reaching a desorption temperature, wherein high concentration values can occur. Due to the inhibited adsorption according to the invention, that is, completely prevented or highly reduced adsorption of NOx, an enrichment of NOx is avoided in the catalytic converter. An increased emission of NO2 due to a desorption is thus inhibited during the heating of the SCR catalytic converter above the desorption temperature and also below the operating temperature. Further, it is avoided by the inhibition of a NOx adsorption at low temperatures according to the invention that a formation of nitric acid and/or nitrous acid takes place in the SCR catalytic converter, which can effect a damage of the SCR catalytic converter or of parts of the exhaust gas purification system on the one hand, and can appear by means of an emission of undesired and highly toxic nitrous gases with a thermal desorption on the other hand. For a reduction of the NOx emission altogether, it is thereby advantageous if a material is used for the adsorption at the mentioned adsorption centers, with which a NO2 reduction or generally a NOx reduction even below the operating temperature is possible for the SCR catalytic converter.
In the arrangement of the method according to the invention, the exhaust gas is enriched with hydrocarbons for preventing an adsorption of nitrogen oxides. An enrichment with low-molecular hydrocarbons is especially preferred. With regard to an effective catalyzation of the selective NOx reduction with NH3 and also a blocking of adsorption centers and/or a low temperature reduction of NOx with the supplied hydrocarbons, a SCR catalytic converter is especially suitable, which contains a solid acid. A dosing with a transition metal as e.g., iron (Fe) or a metal of the platinum group as e.g., platinum (Pt) can be provided additionally. The corresponding adsorption centers are thereby preferably formed as Lewis and/or Brönstedt acid centers. The possibly present transition or platinum metal center can also be effective as an adsorption center for NOx and for hydrocarbons. Such an embodiment of the SCR catalytic converter can also be advantageous with regard to other additives for blocking the NOx adsorption centers.
The object of the invention is also achieved in that, with a lean burning motor vehicle internal combustion engine, with an exhaust gas purification system with a SCR catalytic converter enabled for the reduction of nitrogen oxides (NOx) under oxidizing conditions, and a device for enriching the exhaust gas with ammonia and/or a reagent enabled for splitting off ammonia arranged upstream of the SCR catalytic converter, the total emission of nitrogen oxides of the motor vehicle is decreased below a predeterminable first amount by supplying the SCR catalytic converter with an exhaust gas enriched with ammonia above an operating temperature, and the portion of nitrogen dioxide (NO2) at the total emission of nitrogen oxides (NOx) is decreased below a predeterminable second amount by converting nitrogen dioxide (NO2) with hydrocarbons stored in the SCR catalytic converter.
The predeterminable first and second amounts are preferably an amount of the respective material defined by its mass, which is passed to the environment in a certain predetermined driving distance and/or in a predetermined collective of drive and/or operating states.
The lowering of the NOx total emission thereby takes place mainly or at least to a considerable part by the SCR catalytic converter. For this, an enrichment of the exhaust gas with ammonia takes place above its operating temperature. Thereby, ammonia and/or a reagent for splitting off ammonia are added to the exhaust gas upstream of SCR catalytic converter. An enrichment of the exhaust gas with ammonia can however also take place by a component of the exhaust gas purification system. It can additionally be provided to reduce a portion of the NOx raw emission of the internal combustion engine by one or several other exhaust gas purification units enabled for NOx reduction, which are preferably connected upstream of the SCR catalytic converter in the exhaust gas purification system.
For reducing the portion of nitrogen dioxide NO2) in the total emission of nitrogen oxides (NOx), an exhaust gas enriched with hydrocarbons is preferably supplied to the SCR catalytic converter especially below the operating temperature. The SCR catalytic converter is preferably formed in such a manner that it can adsorb the hydrocarbons increasingly present in the exhaust gas at least below the operating temperature and/or can catalyze a reduction of NO2 with these hydrocarbons. In this manner, a reduction of the emission of NOx in general and especially of NO2 is enabled with a normal operating temperature of the SCR catalytic converter, and also below this temperature.
With a normal operating temperature, that is, above the operating temperature, an NOx reduction takes place by selective reduction at or in the SCR catalytic converter by means of NH3. However, this is not possible (or is possible only to a low degree) below the operating temperature. The inventors have however discovered that especially below the operating temperature, NO2 supplied to the SCR catalytic converter and/or which is adsorbed in the SCR catalytic converter can be can be converted to NO or to N2 by means of hydrocarbons stored in the SCR catalytic converter. The portion of the particularly damaging NO2 at the total emission falls thereby. By means of stored hydrocarbons, for example due to blocked reaction or adsorption centers, a heterogeneously catalyzed NO oxidation to NO2 can also be avoided, whereby the NO2 emission is also reduced. Alternatively or parallel, a low temperature reduction of NOx to N2 can be catalyzed by the SCR catalytic converter with HC present in the exhaust gas in an increased manner. The use of an SCR catalytic converter optimized for the mentioned functions is preferably provided. By means of the method according to the invention, a reduction of the emission of at least the NO2 portion of NOx contained in the exhaust gas can also be achieved below the operating temperature.
In a further arrangement of the invention, the supply of exhaust gas enriched with hydrocarbons to the SCR catalytic converter takes place during a warm-up phase and/or during an idling phase and/or a coasting mode of the internal combustion engine. The operating temperature is not yet reached in these operating states, or a cooling below these can occur. The SCR catalytic converter is thus typically not available for a selective NOx reduction with NH3 in these operating phases and possibly also some time after their completion. If the exhaust gas is enriched with hydrocarbons at least intermittently in these operating phases, the effects mentioned above with regard to undesired NOx or NO2 emission can be avoided or at least diminished. If oxidation catalytic exhaust gas purification components are connected upstream of the SCR catalytic converter, they can still or already be active in the mentioned operating states with regard to an oxidation of NO to NO2, even if the SCR catalytic converter is not above its operating temperature. Due to the enrichment of the exhaust gas with hydrocarbons according to the invention, it is enabled to prevent an adsorptive enrichment of NO2 generated upstream in the SCR catalytic converter and/or to convert generated NO2 to NO and/or N2 by a catalyzed reaction at the SCR catalytic converter with the hydrocarbons and to make them more or less harmless.
In a further arrangement of the method according to the invention, an enrichment of the exhaust gas with hydrocarbons is ended and an enrichment of the exhaust gas with ammonia is started, if, starting from a temperature of the SCR catalytic converter below an operating temperature, the operating temperature of the SCR catalytic converter is exceeded. In this manner, the duration of the enrichment phase and the use of hydrocarbons is kept low and an early reduction of NOx by selective reduction by means of NH3 is ensured.
In a further arrangement of the method, an enrichment of the exhaust gas with hydrocarbons takes place by an incomplete combustion of fuel supplied from the internal combustion engine. This is preferably realized by an afterinjection of fuel into a combustion chamber of the internal combustion engine with an altogether lean air-fuel ratio. The enrichment of the exhaust gas with preferably low molecular hydrocarbons and/or carbon monoxide can however also be achieved by a combustion with an altogether rich lambda value of about 0.95 to 0.8.
In a further arrangement of the method, an enrichment of the exhaust gas with hydrocarbons takes place through a separate supply unit. The supply unit can for example be formed as a pure evaporator unit for evaporating the fuel used by the internal combustion engine. An embodiment as a crack unit for the catalytically supported or thermal cracking, especially with partial oxidation of the fuel is however preferred, as reactive and/or adsorptive highly effective species can be generated with such a supply unit. A reformation running simultaneously with generation of carbon monoxide and/or hydrogen is thereby advantageous.
In a further arrangement of the method, with an exhaust gas purification system with a oxidation-catalytically effective exhaust gas purification component connected upstream of the SCR catalytic converter, starting with a temperature of the SCR catalytic converter below the operating temperature, an enrichment of the exhaust gas with hydrocarbons upstream of the oxidation-catalytically effective exhaust gas purification component is ended when the oxidation-catalytically effective exhaust gas purification component reaches a noticeable activity for the oxidation of hydrocarbons. It is ensured in this manner that hydrocarbons supplied to the exhaust gas are not oxidized partially or predominantly before they reach the SCR catalytic converter. Preferably, the temperature of the oxidation-catalytically effective exhaust gas purification component is monitored by a temperature sensor, and when reaching its start-up temperature with regard to an oxidation of the hydrocarbons, the enrichment is ended. The oxidation-catalytically effective exhaust gas purification component can be an oxidation catalytic converter, a three-way catalytic converter, a nitrogen storage catalytic converter and/or a catalytically coated particle filter.
The enrichment of the exhaust gas with hydrocarbons is ended in a further arrangement of the invention, when the exhaust gas downstream of the SCR catalytic converter has a predeterminable content of hydrocarbons, and/or when the SCR catalytic converter has a predeterminable charge with hydrocarbons. In this manner, a discharge of hydrocarbons to the environment is avoided. For monitoring the hydrocarbon content downstream of the SCR catalytic converter, a lambda sensor or another sensor sensitive with regard to hydrocarbons can be used. It is however also possible to determine by means of known material data of the SCR catalytic converter and the amount of exhaust gas, at which time the catalytic converter has adsorbed a sufficient amount of hydrocarbons. In this manner, an undesired slip of hydrocarbons can also be avoided.
In a further arrangement of the method, an SCR catalytic converter based on zeolite is used. Zeolite catalytic converters have been proven to be especially suitable for the catalyzation of a selective nitrogen oxide reduction by means of NH3 and can furthermore possibly have a more or less strong capability for the adsorption of hydrocarbons and nitrogen oxides.
Advantageous embodiments of the invention are described in the following with reference to a drawing. The above-mentioned characteristics, which are to be explained in the following, can thereby not only be used in the respectively given characteristics combination, but also in other combinations or on their own without leaving the scope of the present invention.